Ports for pressure measurements on ablating surfaces



A. KIRPICH 3, 65, 9

PORTS FOR PRESSURE MEASUREMENTS 0N ABLATING SURFACES Sept. 9, 1969 FiledNOV. 5, 1965 AARON BY A VENI'OR.

KIR P ICH (3.4.1? ATTORN E25 FIG-7 United States Patent 3,465,594 PORTSFOR PRESSURE MEASUREMENTS 0N ABLATING SURFACES Aaron Kirpich, Broomall,Pa., assignor to the United States of America as represented by theSecretary of the Air Force Filed Nov. 5, 1965, Ser. No. 506,917 Int. Cl.G011 7/00 US. Cl. 73-388 7 Claims ABSTRACT OF THE DISCLOSURE Thin wafersor discs of material of high ablation resistance are stacked in acounter-bored hole adjacent the surface of a body whose surface pressureis to be measured. The surface pressure is transmitted through anopening through the center of the stack, and is transmitted to measuringinstruments located inside the body.

The invention relates to ports for pressure measurements on ablatingsurfaces, and more particularly to the formation and design ofpassageways for transmitting static pressure from the surface of bodiessubjected to high heat fluxes to pressure transducers located in theinterior of the body.

The measurement of an aerodynamic static pressure adjacent to thesurface of a particular shape is conventionally obtained with the use ofa small hole drilled perpendicularly to the surface. This passage isdirected to some form of transducer located within the body forreceiving the pressure signal. This method presupposes that the holeopening retains a sharp, well-defined edge. In practice this methodfails in accuracy. The conditions present are extremely hightemperatures and pressures, and failure results from the followingpossible conditions; (1) the plugging of the passage to the transducerby molten products of ablation, and (2) the distortion of thewell-defined edge of the hole by crumbling and erosion of the materialof the aerodynamic body, thereby interfering with the accuracy of thepressure reading.

The object of the present invention is to produce a port design whichwill obviate the above noted difiiculties and render possible moreuniform and more accurate surface pressure determination. To accomplishthis purpose a port must be provided which presents at all times awell-defined and unobstructed opening while the surface around it ismelting and eroding. Thin wafers or discs of material of high ablationresistance are stacked in a counter-bored hole adjacent the surfacealong which static pressure is to be measured. The discs have centrallylocated openings or a hole is drilled through the center of the stackeddiscs to provide a passageway from the surface to a pressure transducer.As ablation, erosion, melting and crumbling of the bulk material of thebody proceeds, individual discs are sloughed off, exposing a sharp cleanhole through which surface pressure conditions are accuratelytransmitted.

Graphite has been found effective for two reasons. First its ablationresistance resistance is high, and second, the lubricating propertypossessed by graphite prevents adherence of molten ablation products tothe local graphite area surrounding the hole. That is, the moltenmaterial is sloughed off and clogging of the hole is thus prevented. Thesuperior resistance of graphite to ablation as compared to the bulkmaterial surrounding it would insure a definite step-by-step sloughingof the discs, helping to prevent the gouging out of the body material,as would be the case where no lining material were present or ifmaterial of poorer ablation resistance were used. Should the hole becomepartially plugged, the eventual 'ice sloughing of the partially pluggeddisc will expose a clear passageway underneath.

These and other advantages, features and objects of the invention willbecome more apparent from the following description taken in connectionwith the illustrative embodiments in the accompanying drawings, wherein:

FIGURE 1 is a perspective view of an ablation type model comprising a 9coned sphere;

FIGURE 2 is a cross section of a passageway lined with graphite discsand mounted in the opening of the ablation type element of FIGURE 1;

FIGURE 3 is a cross section of a closed ended port showing thepassageway lined with graphite discs;

FIGURE 4 is a perspective view of an ablation type model in the form ofa radiused wedge having four ports on its ablating surfaces;

FIGURE 5 is a cross section taken substantially on the line 5-5 ofFIGURE 4;

FIGURE 6 is a cross-sectional view taken substantially on the line 6-6of FIGURE 4; and

FIGURE 7 is .a cross-sectional view of an ablation type model such asshown in FIGURE 1 and showing a pair of passageways leading each to apressure transducer.

Referring more in detail to the drawing, a body member 10 isrepresentative of the ablation type model subjected to high heat andhigh pressure conditions such as are to be found in areas exposed torocket fire. An opening, port, or passageway 12 is shown on the surfaceof the body member 10, to provide means for communicating surfaceconditions to pressure sensing devices. These ports are subject toclogging due to the erosion and melting of the surface of the body. Toprevent this clogging condition, the port or opening 12 is lined with aseries of stacked discs 14. Graphite has been chosen because of itssuperior ablation resistance characteristic and because of itslubricating property. It is conceivable, however, that other materialshaving the required properties can be used.

The dimensions of the graphite discs are of the order of inchdiameter'and .010 inch in thickness. The discs may be washers havingcentrally mating openings or the the port 12 may be constructed byboring an opening through the cylinder of stacked graphite discs 14. Forpurposes of comparison, experiments were conducted with closed boressuch as are shown at 16 in FIGURE 3, and with open bores 18 shown inFIGURE 2 leading directly from the body surface to some sort of pressuresensing device such as the transducers 20 shown in FIG- URE 7.

Various designs for the linings of the passageways have been contrived.In FIGURES 2 and 3, a single stack of graphite discs 14 are placed insurface counter-bores.

In FIGURES 4, 5 and 6 a radiused wedge ablation type model 28 isprovided with ports having graphite linings which are circumferentiallysurrounded by a bushing of ablating material. These ports are designedin two different ways.

In FIGURE 6 a single ring 30 of ablating material provides a lining forthe bore 31 and tapers from the base of the bore to accommodate acomplementary coned stack of discs 35 increasing in diameter from thesurface inward.

In FIGURE 5 a double cone is devised wherein two series of washers ordiscs 22-24 of inwardly increasing size are superimposed along thelength of the passageway. These laminated cones are locked in by asurrounding bushing 34 of material whose ablation resistance is lessthan that of the graphite or other material which comprises the wall ofthe passageways 37. The sloughing off of the discs of each laminatedcone is deterred or slowed down.

Thermocouples 36 and 38 may be located underneath the graphite stacks sothat the temperature as well as the surface pressure may be measured.

Body materials may be phenolic glass or the like, and the plugs andbushings may be phenolic refrasil materials such as fibrous silica ofhigh purity SiO which can withstand extremely high temperatures and areuseful as thermal insulation in conditions such as are encountered injet aircraft, tail cones and pipes.

The sensing devices used may be of the strain type pressure pickups asModel 4-312 of Consolidated Electrodynamics Corporation. These pressurepickups may be fed to oscillograph devices.

It will now be seen that the static pressure present at the surface ofthe bodies 10, 28 and 40 can be transmitted to internally locatedsensing means and measured with a great degree of accuracy since thestacked graphite wafers slough off as the ablating surface of the bodywears away. The graphite lining, which withstands higher temperaturesthan the surrounding ablating surface, wear away and slough off as thesurrounding surface wears, thus preventing clogging and keeping theopening free and unobstructed.

I claim:

1. A device for pressure measurements on ablating surfaces of bodiessubjected to high heat fluxes, said device comprising a body, pressuresensing means located in the interior of said body, said body beingprovided with a bore from said pressure sensing means to the surface ofsaid body, a laminated element located in said bore, said laminatedelement having a central opening therethrough to provide a passagewayfrom the surface of said body to said pressure sensing means, saidlaminated element being of higher ablation resistance than the materialof said body.

2. A device for pressure measurements at ablating surfaces of bodiessubjected to high heat fluxes, said device comprising pressure sensingmeans located in the interior of a body, means for providing sensingcommunication from said sensing means to the surface or said body, saidmeans comprising a passageway, a laminated bushing element in saidpassageway, said laminted bushing element comprising a plurality ofstacked graphite discs.

3. A device according to claim 2 wherein said laminated bushing elementis cone shaped and wherein the diameters of the stacked graphite discsincreases from the surface of said body inwardly.

4. A device for augmenting the accurate measurement of surface pressureson bodies subjected to high heat fluxes, said device comprising apassageway for providing transmission of unchanged pressure states fromthe surface of a body to sensing means located in the interior thereof,a double lining for said passageway, one element of said double liningcomprising a gasket of ablating material having a central bore taperedtoward the surface of said body, the second element comprising a gasket,conical in shape, and fitting within said tapered bore, said gasketbeing formed of stacked graphite wafers of diameters increasing from thesurface of said body toward its interior.

5. A device for allowing accurate and unmodified transmission of surfacepressures from the ablating surfaces of bodies subjected to high heatfluxes to sensing means located in the interior of said bodies formeasurement of said pressures, said device comprising a port in saidbody from the surface thereof to said sensing means, means formaintaining said port free from obstruction by the molten ablatingmaterials of said body, said means comprising a double lining for saidport, one element of said lining comprising a cone shaped stack oflaminated material of high ablation resistance, the second elementcomprising a locking gasket of ablating material surrounding said stack.

6. A port for transmission of surface pressure conditions present at theablating surfaces of bodies subjected to high temperatures and pressuresto pressure sensing means located in the interior of said bodies foraccurate measurement of surface pressures, and means for maintainingsaid port in open condition and free from molten ablating materials,said means comprising a cone of graphite discs providing a lining forsaid port and a locking gasket of ablating material surrounding saidcone.

7. A port for the transmission of surface pressures of ablating bodiesto pressure sensing means located in the interior of said bodies, andmeans for maintaining said ports open and free from molten ablatingmaterials, said means comprising at least two gaskets superimposed oneach other along the length of said port, each gasket comprising alaminated cone shaped stack of discs of ablation resisting material andan element circumferentially surrounding said stack, said element beingmade of ablating material to provide a partial deterrent for sloughingoff of the discs of said laminated cone.

References Cited UNITED STATES PATENTS VERLIN R. PENDEGRASS, PrimaryExaminer

